What are Protists?
These unique and varied organisms demonstrate such unbelievable differences that they are
sometimes called the “junk drawer kingdom”. This kingdom contains the eukaryotes that
cannot be classified into any other kingdom. Most protists, such as the ones shown in
(Figure9.1), are so tiny that they can be seen only with a microscope. Protists are mostly
unicellulareukaryotesthatexistasindependentcells.However,afewprotistsaremulticellular
and surprisingly large.The protists that do form colonies (are multicellular) do not, however,
show cellular specialization or differentiation into tissues. Cellular specialization is a major
featureofmulticellularorganismsabsentintheseprotists. Forexample,kelpisamulticellular
protist and is over 100-meters long.
A few characteristics unify the protists:
- they are eukaryotic which means they have a nucleus
- most have mitochondria
- many are parasites
- they all prefer aquatic or at least moist environments.
For classification, the protists are divided into three groups: animal-like protists, plant-like
protists, and fungi-like protists. But remember they are not animals or plants or fungi, they
areprotists((Figure9.2)). Astherearemanydifferenttypesofprotists, theclassificationof
protists can be difficult. Recently, molecular analysis has been used to confirm evolutionary
relationships among protists. These molecular studies compare DNA sequences. Protists
with higher amounts of common DNA sequences are evolutionarily closer related to each
other. Protists are widely used in industry and in medicine.
Protists Obtain Food
Protists need to perform the necessary cellular functions to stay alive. These include the
need to grow and reproduce, the need to maintain homeostasis, and the need for energy. So
they need to obtain food to provide the energy to enable these functions.
So how are animal-like, plant-like, and fungi-like protists distinguished from each other?
Mainly through how they get their carbon. Of course, carbon is essential in the formation
of organic compounds: carbohydrates, lipids, proteins, and nucleic acids. You get it from
eating, as do other animals.
For such simple organisms, protists get their food in a complicated process. Although there
are many photosynthetic protists (such as the algae discussed in the Plant-like Protists
section below) that get their energy from sunlight, many others still must swallow their food
through a process like endocytosis. Endocytosis was discussed in theCell Functionschapter.